
where the shape functions are defined as [1]}:
\(N^{1}\left(\xi _1,\xi _2,\xi _3\right) & = \xi _1 \\N^{2}\left(\xi _1,\xi _2,\xi _3\right) & = \xi _2 \\N^{3}\left(\xi _1,\xi _2,\xi _3\right) & = \xi _3 \\N^{4}\left(\xi _1,\xi _2,\xi _3\right) & = 1-\xi _1-\xi _2-\xi _3\) 
